The invention relates to a high-frequency ion source for the generation of large-area ion beams. The ion source has a vessel for the substances, particularly gases, which are to be ionized. High-frequency energy can be inductively coupled into the plasma and a weak, d.c. magnetic field is provided.
Large-area ion sources are required for a variety of applications. For instance, they represent an indispensable tool for modern surface and thin film technology. Typical uses, among others, are the large-area removal of homogeneous solid surfaces or solid surfaces covered with structure-producing masks (ion beam etching), or ion beam atomization of one or more solid targets for the production of thin surface layers or thin film systems.
As a rule, the large-area ion sources employed for this consist of a bundle of many individual ion beams which are extracted from a low-pressure plasma by means of one or more sieve-like electrodes arranged one behind the other at a negative potential relative to the plasma. The necessary low-pressure plasma is produced in the respective working gas by electron impact ionization in a known manner. The required electrons are mostly supplied by electron sources of various structure having an electron-accelerating section downstream.
Such ion sources, however, have significant drawbacks during continuous operation or with certain working gases. Thus, for example, thermionic electron emitters ("glow cathodes") have only a limited life. They are very rapidly contaminated or completely destroyed when reactive working gases are used. Due to the limited lateral extent of electron emitters or other electron sources, plasmas of large cross section can be realized only by the use of electron sources which are disposed next to one another. This leads to a non-uniform distribution of the plasma density, and consequently of the ion current density distribution, in the extracted ion beam bundle. Furthermore, as a result of the required current and voltage supplies for the electron sources necessary for plasma production, it is technically complicated to operate such sources at high electrical potential, i.e., to accelerate the ion beam from a high potential onto a grounded workpiece.
The British Patent Specification No. 1 399 603 discloses ion sources in which a d.c. magnetic field continuously acts in axial direction with respect to the vessel axis and, principally by precision utilization of the field inhomogeneity, functions to focus the source plasma in a direction towards the extraction region. A strong magnetic field is used and the vessel always has the same cylindrical form.
Details of an ion source are presented in "Patent Abstracts of Japan", Vol. 10, No. 186 (E-416) (2242), 28 June 1986 and the Japanese Publication No. 61-34832. Thus, by way of example, an ion extraction system with a plurality of extraction electrodes is disclosed in which one electrode can have a suppression voltage.
Details of an ion source are also contained in the French Publication No. 2 359 996 which, among other things, is concerned with an extraction electrode of non-conductive material.